A group-III nitride semiconductor is a semiconductor using nitrogen as a group-V element in group III-V semiconductors. An aluminum nitride (AlN), a gallium nitride (GaN), an indium nitride (InN) are typical examples of the group-III nitride semiconductor. In general, the group-III nitride semiconductor may be expressed as AlxInyGa1-x-yN (where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1).
A high electron mobility transistor (HEMT) using such a nitride semiconductor has been proposed. The HEMT includes, for example, an electron transit layer made of GaN and an electron supply layer made of AlGaN epitaxially grown on the electron transit layer. A pair of electrodes (e.g. source electrode and drain electrode) is formed to be in contact with the electron supply layer, and a gate electrode is disposed therebetween. The gate electrode is disposed to face the electron supply layer with an insulating film disposed therebetween. Due to the polarization caused by the lattice mismatch of GaN and AlGaN, a two-dimensional (2D) electron gas is formed in an inward position by a few Å from an interface between the electron transit layer and the electron supply layer, within the electron transit layer. The source and the drain are connected through the 2D electron gas as a channel. When the 2D electron gas is blocked by applying a control voltage to the gate electrode, the source and the drain are disconnected from each other. In a state where the control voltage is not applied to the gate electrode, the source and the drain are conducted, forming a normally ON type device.
Since a device using a nitride semiconductor has characteristics such as a high voltage, a high temperature operation, a large current density, high speed switching, and a low on-resistance, the applications to power devices have been studied.
However, in order to be used as a power device, since it is required to be a normally OFF type device in which a current is cut off at the time of zero biasing, the aforementioned HEMT cannot be applied to the power device.
A structure for realizing a normally OFF type nitride semiconductor HEMT has been proposed.
An example for achieving a normally OFF scheme has a configuration in which a p-type GaN layer is stacked on an AlGaN electron supply layer, a gate electrode is disposed thereon, and a channel is lost by a depletion layer spreading from the p-type GaN layer.
When a semiconductor device having the above described configuration is manufactured, the p-type GaN layer is formed on the AlGaN electron supply layer and a gate electrode film is formed on the p-type GaN layer. Thereafter, the p-type GaN layer and the gate electrode film are selectively etched, thereby forming a gate part including the p-type GaN layer and the gate electrode. In the above semiconductor device, high precision is required to obtain the proper etching depth during a gate part forming process.
The reason is as follows. When the p-type GaN layer and the gate electrode film are selectively etched, if even the AlGaN electron supply layer is etched, a thickness of the electron supply layer is reduced, and therefore, the polarization of the electron supply layer is reduced to reduce a 2D electron gas density. On the other hand, even when the p-type GaN layer remains in a region other than a region directly below the gate part, the 2D electron gas density is also reduced. That is to say, the 2D electron gas density is significantly varied depending on an etching depth during the gate part forming process. Therefore, a high precision is required to obtain the proper etching depth during the gate part forming process.